Electric machine with low eddy current losses

ABSTRACT

A rotating electric machine having a magnetic circuit which in one of the parts of a rotor and a stator of the machine comprises an element ( 1 ) having a slot for a winding of layers ( 14, 16 ) of cables ( 9 ) extending substantially axially and arranged substantially radially outside each other, said cables comprising an inner conductor ( 10 ) comprising a plurality of strands ( 13 ) and an insulation ( 11 ) arranged outside thereof, has a larger share of strands of the cables closest to the other part of the rotor and the stator electrically insulated ( 15 ) with respect to each other than in the cables in the cable layer ( 16 ) most far away from said other part.

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a rotating electric machine having amagnetic circuit which in one of the parts of a rotor and a stator ofthe machine comprises an element having a slot for a winding havinglayers of cables extending substantially axially and arrangedsubstantially radially outside each other, said cables comprising aninner conductor comprising a plurality of strands and an insulationexternally thereof.

All types of rotating electric machines of the type with winding of acable are comprised, i.e. such machines in which there is an insulationsheet around the conductor and the conductor is formed by a bundle ofstrands. All voltage ranges, high voltage as well as intermediatevoltage and low voltage are comprised.

The element in the magnetic circuit with a slot for the cables may asmentioned be arranged in any of the parts: rotor and stator, of theelectric machine. “Slot” is here to be given a broad sense and does notnecessarily mean that this is so narrow that the element alone keeps thecables in place.

The electric machine may be arranged to function as generator and/ormotor. For the purpose of example it may be mentioned that the machinecould be a synchronous machine used as generator for connection todistribution and transmission networks or as motor or for phasecompensation and voltage regulation. Other types of machines, such asasynchronous alternating current machines are also conceivable.

The element has a design allowing an alternating magnet flux therein,and it is preferably but not necessarily formed by a magnetic core oflaminated sheet being normal or oriented, i.e. thin sheets beingmutually insulated, for example through an insulation lacquer so as tokeep the eddy current losses in the element on an acceptable low level.

A rotating electric machine of the type defined in the introduction isfor example known through WO 97/45919 of the applicant, and it isschematically illustrated in the appended FIG. 1 how an electric machineof that type may be constructed. The element 1 of the magnetic circuitis in this case formed in the stator 2. The rotor with two rotor poles3, 4 shown (it will in the practice have more, for example four) isdesignated by 5. The element 1, or actually the stator, is In aconventional way composed by a laminated core of electric sheetsuccessively composed by sector-shaped plates. The number of teeth 7extends from a back portion 6 of the core located radially outermostradially inwardly towards the rotor. A corresponding number of slots 8are arranged between the teeth. The slots receive a winding of layers ofcables 9 extending substantially axially and arranged radially outsideeach other. The cables 9 comprise an inner conductor 10 consisting of aplurality of strands and an insulation 11 arranged outside thereof.Since we speak about a high voltage generator and the voltage of thecable layers increases with the distance from the rotor through theconnection made here the insulating layers get thicker in the directionaway from the rotor. As a consequence of the limited availability ofsuitable cable dimensions no continuous- decrease of the cableinsulation towards the rotor has taken place, but cables having threedifferent dimensions of the cable insulation are used, such as forexample for 70 kV, 100 kV and 130 kV.

It is illustrated in FIG. 3 how the magnetic alternating flux generatedin the teeth 7 of the element 1 upon rotation of the rotor extendsaround the cables arranged in the slot 8 in question. A stray flux willas illustrated by dashed lines 12 be created through the conductors inan attempt of the magnetic flux to make a shortcut. This stray or leakflux involves some inconveniences. Firstly, the main flux is reducedtherethrough, which results in a somewhat lower power of the electricmachine. Furthermore, the stray flux will generate eddy currents in thestrands, which results in heat generation and a demand of cooling thecables, which normally takes place indirectly by cooling the sheetpackage surrounding them. The strands have been electrically insulatedfrom each other for reducing the eddy current losses, so that themagnetic flux experiences thin surfaces when intersecting the conductorsof the cables and thereby the eddy currents and accordingly the eddycurrent losses will be low. However, this means that the conductors andthereby the cable will be considerably more expensive than if thestrands had been uninsulated, and the insulation is usually achieved bypainting the strands with an insulating lacquer, which means a load onthe environment.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a rotating electricmachine of the type defined in the introduction, in which at least thedisadvantage last mentioned of such machines already known has beensubstantially reduced.

This object is according to the invention obtained by the fact that insuch a rotating electric machine a larger share of the strands of thecables closest to the other part of the rotor and the stator areelectrically insulated with respect to each other than of the cablesmost far away from the other part.

Thus, the invention utilizes the understanding that the magnitude of thestray or leak flux through the respective cable depends upon therelationship between the flow path closed through the cable and thealternative flow path around the cables, which means that the stray fluxdecreases for each cable layer in the direction away from the rotor. Ithas turned out that it is therefore possible to allow considerablylarger continuous surfaces in the cable conductors intersected by themagnetic flux farther away from the rotor than closer thereto andthereby larger eddy current loops, since the stray flux is in any way somuch lower that the eddy current losses in the conductors are kept on anacceptably low level. By electrically insulate fewer strands withrespect to each other in the cables most far away from the other part,in the case discussed above the rotor, than in the cables closer to thisother part, considerable costs may be saved. The costs of a strand withan insulation are normally in the order of twice the costs of a strandwithout insulation. Furthermore, it is when using an insulating lacquerfor the insulation in this way possible to spare the environment by aconsiderably reduced consumption of lacquer when manufacturing thecable. Accordingly, the Invention is based on the idea to concentrate onreducing eddy current losses where it is mostly needed, i.e. where theleak or stray flux is the highest.

According to a preferred embodiment of the invention substantially allstrands are electrically insulated with respect to each other in thecable layer closest to said other part, which is advantageous, since theleak flux is there the highest and the need to keep the surfacesexperienced by this leak flux down on a low level is then also thegreatest.

According to another preferred embodiment of the invention substantiallynone of the strands are electrically insulated with respect to the restof the strands in the cable layer located most far away from said otherpart. Such an advantageous design of the cable in said cable layer is infact possible, since the leak flux of that cable layer is very lowthanks to the short extra way to go for the main flux around the cablelayer in the element with a considerably higher magnetic reluctance.

According to another preferred embodiment of the invention the share ofstrands electrically insulated with respect to the rest of the strandsof the cable decreases in the direction away from said other part. Theadvantages mentioned above of a lack of electric insulation of thestrands with respect to each other where it is in fact not needed ishereby obtained. According to other preferred embodiments of theinvention said decrease may take place for each cable layer in thedirection away from said other part or stepwise after two or more cablelayers having the same proportion or share of strands being electricallyinsulated with respect to each other in the direction away from saidother part.

According to another preferred embodiment of the invention theelectrical insulation of the strands with respect to each other isobtained by providing the respective insulated strand with an insulatingthin envelope surrounding the strand, which according to an embodimentis formed by an insulating lacquer. According to another embodiment theelectrical insulation of the strands with respect to each other isobtained by making such electrically insulated strands of aluminium, thesurface of which is allowed to oxidate for forming an aluminium oxidelayer surrounding the strand. It is in this way possible to do withoutthe insulating lacquer undesirable from the environmental point of view,and aluminium may then advantageously be used for the strands requiringan electrical insulation with respect to each other and for examplecopper of the strands with no need to be electrically insulated withrespect to each other. Thus, the cable or cables most far away from saidother part could advantageously have a conductor formed by strands ofcopper uninsulated with respect to each other, while the cables closestto the other part could have the conductor thereof formed by strands ofaluminium. Accordingly, in the case discussed above of a high voltagegenerator the 70 kV-cables could for example have strands of aluminium,while the 100 kV-cables could have strands of copper.

According to another preferred embodiment of the invention said windingis at least partially formed by a cable in the form of a flexibleelectric conductor having an envelope able to confine the electric fieldgenerated around the conductor. This enables a reduction of electriclosses, which in its turn results in a lower temperature of the cableand the surrounding element, which reduces the need of cooling and makesit possible to construct cooling arrangements possibly existing in amore simple way than without such a design of the cable. The cable maybe made in the form of a flexible cable, which means substantialadvantages with respect to manufacturing and mounting compared to rigidwindings prefabricated and traditionally used until today. Furthermore,the use results in an insulation system with an absence of gaseous andliquid insulating material with the disadvantage adhered theretoobtained in this way.

Further advantages as well as advantageous features of the inventionappear from the other dependent claims and the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a specificdescription of preferred embodiments of the invention cited as examples.

In the drawings:

FIG. 1 is a simplified axial end view of a rotating electric machine ofthe type according to the invention,

FIG. 2 is a perspective view of a part of one end of the stator of theelectric machine according to FIG. 1 during the manufacturing phasethereof,

FIG. 3 is a detail view of a part of the machine according to FIG. 1illustrating the magnetic flux paths,

FIG. 4 is a graph showing the leak magnetic flux through a cableaccording to FIG. 3 in function of the radial distance of the cable fromthe rotor,

FIG. 5 is a view illustrating the construction of a cable particularlysuited to be used in a rotating electric machine of the type accordingto the invention, and

FIGS. 6 and 7 are enlarged detail views of the innermost and theoutermost cable layer in a rotating electric machine of the type shownin FIG. 1 according to a first and a second preferred embodiment of theinvention, respectively.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

It is firstly illustrated in FIG. 2 how the cables 9 are arranged in theslots in the element 1 by threading them thereinto. All cables are notyet in place here.

It is illustrated in FIG. 4 how the leak flux B decreases for eachposition P of the cable 9 away from the rotor, i.e. with increasingdistance from the rotor. The explanation thereto is that therelationship between the leak flux path and the alternative path for themagnetic flux through the element 1 around the cable layer increasescontinuously.

It is illustrated in FIG. 6 how this understanding has resulted in afirst preferred embodiment of the invention, in which the strands 13(the entire conductor 10 is of course filled by strands, even if thefigures show other things for the sake of simplicity) of the cable layer14 located closest to the rotor are electrically insulated with respectto each other through a thin insulating layer 15 surrounding eachstrand, which may be of a conventional insulating lacquer. This isimportant for keeping the eddy current losses on a low level, since theleak flux is the highest there. However, the strands 13 of the cablelayer 16 located most far away from the rotor are notelectrically-insulated with respect to each other, which is possiblesince the leak flux there is so low that the eddy current losses arestill kept on an acceptably low level. It is by this possible to savefinancial as well as environmental resources. The strands of theoutermost cable may namely be obtained to nearly half the cost withrespect to the strands of the innermost cable. It is possible to designthe cables between the two extreme cable layers 14, 16, so that theshare or proportion of strands electrically insulated with respect toeach other decreases for each cable layer or stepwise after two, threeor the like subsequent cable layers having the same proportion ofinsulated strands in the direction away from the rotor. However, it isalso possible to have for example all strands electrically insulatedwith respect to each other for a certain number of cable layers, forexample half or two thirds thereof, and then have all strandsuninsulated for the rest of the cable layers.

A second preferred embodiment of the invention is illustrated in FIG. 7,which differs from the one according to FIG. 6 by the fact that theelectrical insulation of the strands with respect to each other has beenobtained by using aluminium as material for the strands and allowing thesurface the strands to oxidate for forming an aluminium oxide layer 15surrounding the respective strand. In the case that the strands have notbeen insulated copper have been used for the strands, since this is moreadvantageous through the higher electrical conductivity of copper. Thus,as shown in this Figure, all strands of the innermost cable layer may beof aluminium, while the strands of the outer cable layer 16 are made ofcopper. The advantage of proceeding in this way for obtaining theelectrical insulation of the strands with respect to each other wherethis is found to be necessary is that the insulating lacquer unpleasantfrom the environmental point of view does not have to be used. Costs forthe strands, especially for the one of copper, are simultaneously savedin the same way as in the embodiment according to FIG. 6.

Finally, the construction of a cable of the type particularly wellsuited to be used in a rotating electric machine of the type accordingto the invention at high voltages is illustrated in FIG. 5, especiallyin a high voltage generator according to WO 97/45919 discussed furtherabove. This cable has an inner electric conductor 10 with an envelope 11able to confine the electric field generated around the conductor. Thiscable has an inner flexible electric conductor 10 and an envelope 11forming an insulation system, which comprises an insulation 17 formed bya solid insulation material, preferably a material on polymeric basis,and an outer layer 18 having an electrical conductivity being higherthan the one of the insulation so that the outer layer throughconnection to ground or otherwise to a comparatively low potential willbe able to on one hand operate to equalize potential and on the otherprimarily enclose the electric field created as a consequence of saidelectric conductor 10 interiorly of the outer layer 18. Furthermore, theouter layer should have a resistivity being sufficient for minimizingthe electric losses in the outer layer. The insulation system alsocomprises an inner layer 19, which has said at least one electricconductor 10 arranged interiorly thereof and has an electricalconductivity being lower than the one of the electric conductor butsufficient for making the inner layer to operate for equalizingpotential and thereby act equalizing with respect to the electric fieldoutside the inner layer. Thus, such a cable is of a type correspondingto cables having a solid extruded insulation and today being used withinpower distribution, for example so called PEX-cables or cables withEPR-insulation. The term “solid insulation material” used means that thewinding has to be without any liquid or gaseous insulation, for examplein the form of oil. The insulation is instead intended to be formed by apolymeric material. Also the inner and outer layers are formed by apolymeric material, although a semiconducting one. The insulation 18 maybe made of a solid thermoplastic material, such as low-densitypolyethylene (LDPE), high-density polyethylene (HDPE), polypropylene(PP), polybutylene (PB), polymetylpentene (PMP), cross-linkedpolyethylene (XLPE) or rubber such as ethylene-propylene rubber (EPR) orsilicon rubber. With respect to the resistivity of the inner layer andthe outer layer this should be within the range 10⁻⁶ Ωcm-100 kΩcm,suitably 10⁻³-1000 Ωcm, preferably 1-500 Ωcm. The inner and the outerlayers have advantageously a resistance which per length meter of theconductor/insulation system is in the range 50μΩ-5 MΩ.

The electric load or stress on the insulation system is reduced as aconsequence of the fact that the inner and outer layers of thesemiconducting materials around the insulation will tend to formsubstantially equipotential surfaces and the electric field in theinsulation will in this way be distributed comparatively homogeneouslyover the thickness of the insulation.

The adherence between the insulation material and the inner and outersemiconducting layers has to be uniform over substantially the entireinterface thereof, so that no hollow spaces, pores and the like may becreated. This is of course particularly important in high voltageapplications, and a cable of this type has preferably an insulationsystem adapted for high voltage, suitably over 10 kV, especially over 36kV and preferably over 72.5 kV. The electrical and thermal stressesoccurring at such high voltages make high demands on the insulationmaterial. It is known that so-called partial discharges, PD, is ingeneral a severe problem for the insulation material in high voltageapplications. Should hollow spaces, pores or the like be formed in aninsulating layer, inner corona discharges may occur at high electricvoltages, whereby the insulation material is gradually degraded and theresult could be electrical breakdown through the insulation. This couldresult in a severe breakdown of the reactor.

It is advantageous that the inner and outer layers and the solidinsulation have substantially the same thermal properties for avoidingthe generation of such hollow spaces or pores, in which it isparticularly important that they have substantially the same coefficientof thermal expansion, so that a perfect adherence between the differentlayers may be maintained during temperature changes thereof and thecable expands and contracts uniformly as a monolithic body upontemperature changes without any destruction or degradation of theinterfaces. The insulation layer is for example a PEX-cable ofcross-linked low-density polyethylene and the semiconducting layers ofpolyethylene with dust and metal particles admixed. Volume changes as aconsequence of temperature changes are absorbed entirely as changes ofthe radius of the cable and thanks to the comparatively small differenceof the coefficients of thermal expansion of the layers with respect tothe elasticity of these materials, the radial expansion of the cable maytake place while avoiding that the layers will get loose from eachother.

The cable has also to have a certain flexibility, and it is flexibledown to a radius of curvature below 25 times the diameter of the cableso that bending may take place while ensuring a good adherence betweenthe respective layers and the solid insulation. The cable is suitablyflexible to a radius of curvature below 15 times the diameter of thecable, and preferably to a radius of curvature below 10 times thediameter of the cable. The E-modulus of the different layers in theinsulation system should be substantially equal so as to not induce anyunnecessary shearing stresses in the interfaces between the differentlayers, so that a reduction of the shearing stresses that may be createdbetween the different layers when exerting the cable to powerful bendingresulting in tension stresses on the outside of the bend and compressivestresses on the inside of the bend may take place.

The invention is of course not in any way restricted to the preferredembodiments described above, but many possibilities to modificationsthereof will be apparent to a man skilled in the art without departingfrom the basic idea of the invention as defined in the appended claims.

It would for example be well possible to use other materials than thosementioned above for electrically insulating the strands with respect toeach other, and it would also be possible to imagine differentcombinations of electrical insulation therebetween, so that for examplewhere the leak flux is not that high for example an electricallyinsulating layer is arranged around a smaller amount of strands, as forexample three, for electrically insulating them with respect to anothersuch bundle of strands and obtain a suitable restriction of the size ofthe surface experienced by the leak flux.

For making the electric machine described further above function athigher voltages it is essential that at least one strand of the cable isin electric contact with the inner semi-insulating layer for forming aequipotential surface, and “substantially all strands are electricallyinsulated with respect to each other” in the claims is intended to coverthis case too.

What is claimed is:
 1. A high voltage rotating electric machinecomprising: a rotor; a stator having a slot; and a winding having aplurality of cable layers that each include an inner conductor with aplurality of strands and an insulation disposed about said innerconductor, said plurality of cable layers arranged substantially axiallythrough said slot and substantially radially outside one another,wherein said plurality of cable layers includes an inner cable layer andan outer cable layer, said inner cable layer disposed in said slotradially closer to said rotor than said outer cable layer, a largernumber of strands in said inner cable layer are electrically insulatedfrom one another than strands in the inner conductor of said outer cablelayer.
 2. A high voltage rotating electric machine according to claim 1,wherein: said inner cable layer is an innermost cable layer with regardto a radial proximity to said rotor, and wherein substantially all ofthe plurality of strands of the innermost cable layer include anelectrical insulation thereabout.
 3. A high voltage rotating electricmachine according to claim 1, wherein: said outer cable layer is anoutermost cable layer with regard to a radial proximity to said rotor,and substantially none of the plurality of strands of the outermostcable layer include an electrical insulation thereabout.
 4. A highvoltage rotating electric machine according to claim 1, wherein: aportion of strands insulated from one another in the inner conductor ofrespective of the plurality of cable layers decreases on a cable layerby cable layer basis as a distance to the rotor increases.
 5. A highvoltage rotating electric machine according to claim 1, wherein: aportion of the plurality of strands configured to have an electricalinsulation within each layer of the plurality of cable layers decreasesas a distance to the rotor increases on a cable layer by cable layerbasis.
 6. A high voltage rotating electric machine according to claim 1,wherein: a portion of the plurality of strands configured to have anelectrical insulation thereabout decreases as a distance to the rotorincreases by two or more layers of the plurality of cable layers, wheresaid distance is on a cable layer by cable layer basis.
 7. A highvoltage rotating electric machine according to claim 1, wherein: aplurality of slots are arranged in the stator.
 8. A high voltagerotating electric machine according to claim 1, wherein: a circuitformed between the stator and rotor is configured for high voltageoperation; the plurality of cable layers are configured to have anelectric potential developed therein that increases as a distancebetween the winding and the rotor increases; and an insulation thicknessof the insulation decreases in at least one of a continuous manner and astepwise manner as a distance from the rotor increases on a cable layerby cable layer basis.
 9. A high voltage rotating electric machineaccording to claim 1, wherein: a strand that is insulated in the innercable comprises aluminum, and an insulation on the strand is an aluminumoxide.
 10. A high voltage rotating electric machine according to claim1, wherein: the insulation comprises a thin electrically insulatingenvelope.
 11. A high voltage rotating electric machine according toclaim 10, wherein: the thin electrically insulating envelope comprisesan insulating lacquer.
 12. A high voltage rotating electric machineaccording to claim 10, wherein: a portion of the plurality of strands insaid inner cable layer that are insulated comprises aluminum and aportion of the plurality of strands in the inner cable layer that areuninsulated are comprised of copper.
 13. A high voltage rotatingelectric machine according to claim 1, wherein: said inner cable layerbeing an innermost cable layer with regard to a radial proximity to saidrotor, substantially all of the plurality of strands are comprised ofaluminum; and said outer cable layer is an outermost cable layer withregard to a radial proximity to said rotor, substantially all of theplurality of strands comprises copper.
 14. A high voltage rotatingelectric machine according to claim 13, wherein: the winding isconfigured to carry voltages greater than 10 kV.
 15. A high voltagerotating electric machine according to claim 14, wherein: the winding isconfigured to carry voltages greater than 36 kV.
 16. A high voltagerotating electric machine according to claim 15, wherein: the winding isconfigured to carry voltages greater than 72.5 kV.
 17. A high voltagerotating electric machine according to claim 13, wherein: the winding isconfigured to be connected to a voltage greater than 10 kV.
 18. A highvoltage rotating electric machine according to claim 17, wherein: thewinding is configured to be connected to a voltage greater than 36 kV.19. A high voltage rotating electric machine according to claim 18,wherein: the winding is configured to be connected to a voltage greaterthan 72.5 kV.
 20. A high voltage rotating electric machine according toclaim 13, wherein: the winding comprises a flexible electric conductorand a casing configured to contain an electric field generated aroundthe flexible electric conductor.
 21. A high voltage rotating electricmachine according to claim 20, wherein: the casing comprises aninsulation system having an inner layer disposed on the flexibleelectric conductor, a solid insulation layer disposed on the innerlayer, and an outer layer disposed on the solid insulation layer,wherein the outer layer is configured to have an electric conductivityhigher than that of the solid insulation layer and is connected to anode having at least one of a ground potential and a low voltagepotential so as to contain the electric field formed around the flexibleelectric conductor.
 22. A high voltage rotating electric machineaccording to claim 21, wherein: the inner layer is configured to have anelectric conductivity lower than the flexible electric conductor and tosubstantially equalize an electric field formed on an outer surface ofthe inner layer.
 23. A high voltage rotating electric machine accordingto claim 21, wherein: the inner layer, the outer layer and the solidinsulation layer are configured to have an essentially equal thermalcharacteristic.
 24. A high voltage rotating electric machine accordingto claim 21, wherein: the inner layer and the outer layer comprise asemiconductor material.
 25. A high voltage rotating electric machineaccording to claim 21, wherein: the inner layer and the outer layer areconfigured to have a resistivity in an inclusive range of 10⁻⁶ Ω-1000Ωcm.
 26. A high voltage rotating electric machine according to claim 21,wherein: the inner layer and the outer layer are configured to have aresistivity in an inclusive range of 10⁻³ Ωcm-1000 Ωcm.
 27. A highvoltage rotating electric machine according to claim 21, wherein: theinner layer and the outer layer are configured to have a resistivity inan inclusive range of 1 Ωcm-500 Ωcm.
 28. A high voltage rotatingelectric machine according to claim 21, wherein: the inner layer and theouter layer are configured to have a resistance in an inclusive range of50 μΩ/m-5 MΩ/m.
 29. A high voltage rotating electric machine accordingto claim 21, wherein: the solid insulation layer and at least one of theinner layer and the outer layer comprise a polymeric material.
 30. Ahigh voltage rotating electric machine according to claim 21, wherein:the solid insulation layer, the inner layer and the outer layer areconfigured to ensure adherence upon flexing and temperature change. 31.A high voltage rotating electric machine according to claim 21, wherein:the solid insulation layer, the inner layer and the outer layer comprisematerials with high elasticity.
 32. A high voltage rotating electricmachine according to claim 21, wherein: the solid insulation layer, theinner layer and the outer layer are comprised of materials with asubstantially equal E-modulus.
 33. A high voltage rotating electricmachine according to claim 21, wherein: the solid insulation layer, theinner layer and the outer layer are comprised of materials with asubstantially equal thermal expansion coefficients.
 34. A high voltagerotating electric machine according to claim 21, wherein: the innerlayer is configured to be in electrical contact with the flexibleelectric conductor.
 35. A high voltage rotating electric machineaccording to claim 21, wherein: the flexible electric conductorcomprises a plurality of strands, and at least one portion of at leastone of the plurality of strands is uninsulated and configured to be incontact with the inner layer.